1. Field of the Invention
The present invention relates to systems and methods for designing and operating feedback control systems, and in particular to a system and method for robustly controlling spacecraft.
2. Description of the Related Art
Controllers are often employed to stabilize or improve the stability of an inherently unstable or marginally stable dynamic system. Such controllers are typically designed using well known state-space or Laplace transform techniques.
However, most dynamic systems include one or more non-linearities (e.g. actuator saturation, sensor dynamic range, plant operation envelope) that make the derivation of closed form controller solutions difficult or impossible.
In most cases, the system non-linearities do not substantially affect controller design or system performance in measures of interest, and can simply be ignored. However, in many cases, system non-linearities have a substantial effect on system performance. In such cases, the system non-linearities must be accounted for in the controller design.
One solution to this problem is to generate a linearized model of the system (including the system non-linearities), and design the controller using linear control system design techniques. However, while appropriate for some non-linearities, this technique is not generally applicable to all non-linearities.
Another solution is to generate a detailed simulation (e.g. an N degree of freedom, or N DOF simulation) of the system, including the non-linear elements, and use that simulation to design a controller to meet system requirements. This solution, however, can be time and resource intensive, both in generating the simulation and in performing multiple monte-carlo simulations to account for stochastically-modeled processes. Detailed simulations can also be very sensitive to modeling errors and uncertainties. Further, even when a detailed N DOF simulation is available, it can be difficult to design a controller without a starting point that is reasonably close to a stable solution that meets system requirements.
These difficulties are particularly apparent when considering the design of a controller for the attitude control system (ACS) of a spinning spacecraft during a transfer orbit mission. During the transfer orbit mission, the spacecraft can be difficult to control due to a strong interaction between the dynamics of fuel sloshing around in the fuel vessels (fuel slosh) and the dynamics of the spinning spacecraft. Fuel slosh dynamics are not only difficult and complicated to model, they are also highly uncertain.
Existing spacecraft ACS transfer orbit controller designs include Wheel Gyro Wobble and Nutation Control (WGWANC) systems. Unfortunately, this design is not robust to fuel slosh dynamics and system uncertainties. This is due to the nature of the WGWANC design methodology, which attempts to decouple 3-axis dynamics by absorbing the inverse of the plant inertial into the controller. This seriously degrades ACS controller robustness when the actual plant dynamics differ from model predictions.
WGWANC design procedures are also typically based on a classical 2nd order approximation of the plant dynamics. The controllers that result require substantial tuning (in the form of gain-scheduling, for example) throughout the mission to assure stability and adequate performance. Such gain scheduling can result in complicated software and mission operation procedures.
What is needed is a simple, effective method for designing system controllers that are robust to system uncertainties and non-linearities, for example, an ACS controller for spinning spacecraft that is robust to fuel sloshing effects.
The present invention satisfies that need.